Since r1303 and WPMU 2.7.1-beta1, BuddyPress has been considered a single plugin that has multiple components. Once WPMU is upgraded, you install BuddyPress.

Then, you log in as site admin in WPMU backend and go to” Plugins > Installed” and activated BuddyPress. Once it’s activated, a new menu group is created at the bottom of the menu tree called “BuddyPress”.

You click on “BuddyPress > Component Setup” and you should see that all the BP components are enabled by default. It is in this admin screen where you can enable and disable individual components.